1. Field of the Invention
This invention relates in general to disk drives, and more particularly to a method and apparatus for performing surface analysis of a recording surface.
2. Description of Related Art
In a conventional magnetic storage drive, an air bearing slider supports a magnetic transducer in close proximity to a relatively moving recording surface. The recording surface typically comprises a rigid disk coated with a layer of magnetic material applied by a method such as spin-coating or sputtering. Coated disks must be substantially free of asperities to assure long-term reliability and data integrity at the head to disk interface, since asperities can lead to undesirable slider-disk contact or "head crash".
Glide height testing is one means for assuring an asperity-free disk. A slider is flown over the recording disk at a height equal to or below the desired data head fly height to analyze impacts between the slider and the disk surface. The slider includes one or more piezoelectric sensors bonded to an upper surface facing away from the recording surface. Piezoelectric materials are used because they generate an electric charge in response to internal stress. As the slider experiences rigid body displacement and flexural deformation as a result of contact with asperities, the adjacent sensor responds by generating a charge signal which may be monitored.
A dominant practice in the art has been to monitor the low frequency piezoelectric signals which corresponds to rigid body displacement resulting from the slider contacting large asperities on the disk surface. But as sliders decrease in size, magnetic transducers become vulnerable to relatively small asperities. For example, there is a class of asperities (e.g. disk delaminations) that are too small to cause head crashes, yet large enough to result in slider-disk contact adversely affecting device reliability. This class of asperities generates high frequency vibrations in the test slider which are difficult to detect.
The optimal sensitivity to small disk asperities is obtained by monitoring the high frequency vibrations of a test slider. Yet the high frequency components, or bending mode frequencies, of the response signal may vary greatly. Many modes display a non-monotonic response with increasing asperity interference height, i.e. the distance between the tip of an asperity and the minimum slider fly height. Non-monotonic modes indicate the occurrence of disk contact but provide no useful information about the size of the asperity causing contact.
The trend in recent years has been to produce storage systems having smaller sliders than the conventional slider which were approximately 4 mm long by 3.2 mm wide. Reductions in slider size necessitate a corresponding reduction in test slider dimensions. This reduction results in a weaker piezoelectric signal and poor signal-to-noise (S/N) ratio. Furthermore, the S/N ratio has also been shown to decrease with decreasing glide height. Thus, optimizing test slider sensitivity becomes increasingly important for smaller slider designs.
It is therefore desirable for a slider of predetermined size and fly height to identify one or more high frequency bending modes displaying monotonic behavior with increasing asperity interference. To that end, it becomes necessary to analyze the various bending mode frequencies individually. One method for isolating bending mode components is to electronically filter the high frequency signal generated by the piezoelectric sensor. But such filtering requires several filtering stages and becomes difficult with low S/N ratios.
An IBM Technical Disclosure Bulletin article entitled "Efficient Piezoelectric Glide Transducer for Magnetic Recording Disk Quality Assurance", Vol. 34, No. 4A, September 1992, describes a test slider comprising two piezoelectric transducers disposed on the upper surface of a slider about its longitudinal axis. Each half is oppositely poled with respect to the other. The arrangement increases the sensor's sensitivity to three low-frequency bending modes indicative of slider rigid body motion. However, detection of the high frequency bending modes is not discussed.
To realize the maximum advantage of advanced glide heads which are suitable for ultra-low flying and contact recording file products, advanced designs are needed. Another design is disclosed in IBM Technical Disclosure Bulletin article entitled "Advanced Piezoelectric Glide Channel", Vol. 38, No. 06, June 1995. To advance the current state-of-the-art in piezoelectric transducer (PZT) channels, this channel combines low noise amplification, high common-mode noise rejection, ground-noise isolation, and dual bandpass slider rigid and slider bending mode selection through filtering for use with slider-mounted PZT sensors.
The piezoelectric glide height sensor channel design/configuration provides for a monotonic sensor response with asperity interference height for signals derived from the slider rigid body modes, as well as selected slider bending modes. This combined channel is optimized for improved signal to noise and dynamic range for the detection of asperities on magnetic recording disks used in direct access storage devices. The low frequencies (slider air bearing modes) are required to detect large diameter (&gt;15 microns) defects, and the high frequencies (slider bending modes) are required to detect small (&lt;15 microns) defects. It is necessary to reject undesirable modes in between these two bands.
An example of the disclosed channels is one optimized for a small slider of 2.5 mm.times.1.6 mm.times.0.43 mm, where the desired modal response is the 5th and 7th modes between 1.0 and 1.4 MHz. The PZT crystal is configured such that the signal from these modes appears differentially between the front and back sections of the crystal. The channel consists of two main components, or physical sections. The first is a differential current (transconductance) amplifier (although a charge amplifier may also be used), and the second section contains the filters and buffers. The filters may include a low frequency filter, e.g., 25-200 kHz, and a high frequency filter, e.g., 1.0 to 1.4 MHz. A detector is then applied to the outputs of the filters to detect defects on the disk. One factor that makes this sensor productive is that the channel is monotonic with increasing defect size, and can be calibrated with laser melt bumps.
However, new files are being developed which have a nominal data head mechanical fly-height of .ltoreq.27 nm. As a result small disk defects can escape the current glide test, and cause unacceptable hard errors in the file.
It can be seen that there is a need for an enhanced glide test that is more sensitive to smaller asperities.
It can also be seen that there is a need for an enhanced glide test that maintains the positive characteristics of being monotonic and which can be calibrated.